The invention relates generally to fiber optics and more specifically to methods of characterizing the polarization mode dispersion in a fiber optic transmission system.
Polarization mode dispersion (“PMD”) generally refers to variations in the time delay of a polarized optical signal traveling through an optical transmission system, such as a single-mode optical fiber. PMD arises in an optical fiber because of asymmetries in the optical fiber core, such as, for example, core ellipticity created during optical manufacturing and bending stresses resulting from handling or formed during installation. The asymmetries of the fiber core cause random changes in the state of polarization (“SOP”) of optical signals propagating through the fiber. Different SOPs propagate through the optical fiber core at different relative speeds, for example, some SOPs travel faster and some travel slower, resulting in a pulse width distortion of a transmitted optical signal relative to an input optical signal. Additionally, the asymmetries of the core are highly susceptible to environmental fluctuations, such as temperature or movement of the fiber, which occur as fast as milliseconds and result in a time varying pulse width distortion of the transmitted optical signal.
First order PMD refers to a time delay between two orthogonally polarized principal states of polarization (“PSP”). The PSPs form a convenient basis set to describe and characterize each SOP, and evaluate the effects of PMD in the fiber. Using the PSPs as a basis set, each SOP propagating through an optical fiber is represented by a linear combination of the two orthogonally polarized PSPs. The varying pulse width distortion of the SOP is a function of a varying delay between the PSPs. Theoretically, each PSP experiences a time of flight difference through the optical fiber, commonly known as differential group delay (DGD), resulting in a time delay between the two PSPs at the fiber output. The output SOP is represented by a linear combination of the PSPs that are time delayed with respect to each other. A greater time delay between the PSPs corresponds to a larger relative difference between the input SOP pulse width and the output SOP pulse width.
Optical fibers have a differential group delay (“DGD”) between the two PSPs on the order of 0.1 ps/(km)1/2. In older fiber optic cables, such as the cables used in terrestrial networks, the DGD is on the order of 2.0 ps/(km)1/2 and results in time delays of about 50 picoseconds for transmission distances of only several hundred kilometers.
Second order PMD refers to the frequency dependence of the first order PMD. This is related to chromatic dispersion of signals in optical fibers, which arises due to the frequency dependence of the refractive index of the fiber. Second order PMD causes the dispersion of the PSPs, which further distorts the optical signal propagating in the fiber.
As the demand for faster optical data transmission increases, such as from gigabits per second to terabits per second, optical pulse width distortion due to PMD may become one of the factors limiting data transmission rate.